Combination electromagnet-permanent magnet focusing devices



Sept. 20,1955 s. ZUERKER 2,718,606

COMBINATION ELECTROMAGNET-PERMANENT MAGNET FOCUSING DEVICES Filed Aug.2, 1952 Inventor: Siegfried Zuerker;

His Attorney.

United States Patent COMBINATION ELECTROMAGNET-PERMANENT MAGNET FOCUSINGDEVICES Siegfried Zuerker, Pennellville, N. Y., assignor to GeneralElectric Company, a corporation of New York Application August 2, 1952,Serial No. 302,313 10 Claims. (Cl. 313-76) My invention relates tostructures for focusing electron beams in electron discharge devices ofthe cathode-ray type, and more particularly to focusing structuresemploying both permanent magnets and electromagnets as sources ofmagnetomotive force.

In cathode-ray tubes, such as are employed in television receivers,Oscilloscopes, and the like, it is generally necessary to provide meansfor centering and adjusting the electron beam in order to orientproperly the image traced by the electron beam on the picture screen. Itis common practice to accomplish this function by means of a magneticfocusing device. This may be of the electromagnetic type, employing asolenoid coil positioned externally around the neck of the cathode-raytube, or of the permanent magnet type, having a toroidal structurearranged to produce a generally cylindrical magnetic field along thebeam axis.

One problem encountered in the electromagnetic type of focusing deviceis that of drift, a gradual defocusing due to fluctuations in coilcurrent caused by resistance variations as a result of heat generatedtherein. These fluctuations have been reduced by providing, in additionto the electromagnetic coil, a permanent magnet in cooperationtherewith, thereby reducing the flux demand on the electromagnet, andresulting in lower current requirements for the electromagnet. Suchcombination focusing devices further tend to stabilize the focus bycompensating for line voltage variations.

In combination electromagnet-permanent magnet focusing devicespreviously known, the arrangement of the magnets around the neck of thetube has generally been such, e. g. concentric, that the fluxes of bothmagnets pass through the permanent magnet structure. In such case, whenthe coil is energized, the flux which it produces through the permanentmagnet, causes the latter to change its operating point on thede-magnetization curve along a minor hysteresis loop. Since the magneticmaterial in the permanent magnet is usually of high reluctance, theelectromagnet is therefore required to supply an appreciable amount ofmagnetomotive force to increase the flux through the permanent magnetand, in addition, to supply the increased power needed to produce theflux in the air gap. It becomes impractical, therefore, to allow theelectromagnetic component of the focusing field to exceed approximatelyten percent of the total required flux. This in turn results in adecrease of efliciency which ordinarily necessitates increasing thecross-section of the permanent magnet over that which is otherwiserequired to provide the necessary field. This correction results in apermanent magnet that is heavier and bulkier and that requires moremagnetic material than would otherwise be necessary.

'In accordance with one aspect of my invention, an electron-beamfocusing apparatus is provided which combinesthe electromagnetic type offocusing means and the permanent magnet type of focusing means in polar.opposed relation. More specifically, it comprises an electron-beamfocusing lens including electromagnetic focusv focusing structure ingmeans and permanent magnet focusing means axially aligned with oneanother in successive relation. These means are arranged so thatmagnetic poles of like polarity face one another and so that each ofsaid magnet means maintains a flux pattern across a separate anddistinct gap, thus excluding said permanent magnet means from the fluxpath of said electromagnetic means.

Thus, it is a main object of my invention to provide an improvedelectron-beam focusing apparatus having both permanent magnet andelectromagnet focusing means wherein the efiiciency of the electromagnetis not aifected by the permanent magnet.

Another object of my invention is to provide a more eflicientelectron-beam focusing apparatus of the combinationelectromagnet-permanent magnet type than has heretofore been achieved,wherein a reduced quantity of magnetic material is required, therebyefiecting a saving of size, weight and cost of the apparatus.

For additional objects and advantages, and for a better understanding ofthe invention, attention is now directed to the following descriptionand accompanying drawings. The features of the invention which arebelieved to be novel are particularly pointed out in the appendedclaims.

Referring to the drawings, Fig. 1 shows a cross-sectional view of acathode-ray tube incorporating a magnetic. embodying the invention; Fig.2 is a perspective view of a magnetic focusing structure embodying theinvention; Fig. 3 is an enlarged view of a longitudinal cross sectionthrough the magnetic focusing structure of Fig. 2; and Fig. 4 is aperspective view of .a modified magnetic focusing structure embodyingthe invention.

Referring now to Fig. 1, there is shown a conventional cathode-ray tubeassembly including an electronbeam focusing lens embodying one form ofthe invention. The cathode-ray tube is provided with a glass en- 'velopecomprising a bowl portion 11 and an elongated cylindrical neck 12.Within the neck 12 near the end farthest from the bowl 11 there isprovided an electron gun 13, the construction and operation of which arewell known in the art. The gun 13 projects a beam of electrons 14 alongthe axis of the neck 12 and against the fluorescent screen 15, causingit to emit light in a well known manner.

A conventional deflecting coil assembly 16 is mounted on the neck 12adjacent the bowl 11 to provide vertical and horizontal beam deflectionin order to provide means for scanning the screen 15 to produce animage, in a manner well understood in the art. Although magneticdeflecting coils have been shown, it is apparent to those skilled in theart that electrostatic deflecting means or combinationelectrostatic-magnetic deflecting means may be employed instead.

In accordance with the invention, an electron beam focusing lens 17 isalso mounted on the neck 12 at a position between the gun 13 and thedeflecting coils 16. Its function is to produce an axial magnetic fieldof proper intensity to cause the beam of electrons 14 to come to a sharpfocus in the form of a spot on the screen 15 and to center the spot thusformed.

As shown in Fig. 2, focusing lens 17 comprises an electromagnetcomponent shown generally at 18 and a permanent magnet component showngenerally at 19. As is illustrated more clearly in the cross-sectionalview of Fig. 3, the lens is provided with a generally toroidal coil 20which is supplied with electric energy by connecting it to someconvenient direct current source (not shown) in order to develop anelectromagnetic field along its axis in a conventional manner. Coil 20is provided with a generally toroidal housing 21 completely surroundingit except foraxially-extending annular gap 22 concentric'with the coil20. Housing 21 is made of-low reluctance material, such as soft iron,which provides a low-resistance path for the electromagnetic fluxsustained by the coil 20 and concentrates the entire electromagneticflux across the gap 22 in well known manner, as represented by thedashed fiux lines 23 in Fig. 3.

The permanent magnet field of lens 17 is provided by a permanent magnet24 in the form of a cylinder that is axially aligned with coil 20 andhousing 21 and magnetized to produce an axial magnetic field of oppositepolarity, as indicated by dashed flux lines 27. In the form of theinvention shown in Fig. 3, the north pole of electromagnetic coil 20faces the north pole of permanent magnet 24 and the north end of magnet24 abuts against housing 21. However, in some cases it may be desirableto place the south poles of the respective magnets in opposition, ratherthan the north poles as illustrated. It may, likewise, be desirable insome cases not to have the opposing members in abutting relationship,providing that a portion of the flux from each flows through a commonpole piece. As shown, annular pole piece 25, formed of low reluctancematerial, such as soft iron, is axially aligned with permanent magnet 24and abuts against the end thereof remote from housing 21. It is apparentto those skilled in the art that a path of low resistance to magneticflux is thus provided for the flux of permanent magnet 24 through polepiece 25 and a portion of housing 21 within the circumference ofpermanent magnet 24. The last-mentioned portion of housing 21 thereforeforms a common pole piece for both electromagnet 20 and permanent magnet24 and, together with pole piece 25, tends to concentrate the entireflux of permanent magnet 24 across the gap 26 as represented by the fluxlines 27.

It is seen from the above that although a part of housing 21 is commonto the flux circuits of both the electromagnet 20 and the permanentmagnet 24, the flux of the electromagnetic circuit does not pass throughthe permanent magnet 24 and, hence, it is unnecessary to consider thereluctance of the cylindrical permanent magnet 24 in determining itssize relative to that of the electromagnet 20.

Referring again to Fig. 1, when an electron beam 14 is projected throughlens 17, it comes under the influence of the two magnetic fields insuccession. First, it is subjected to the focusing action of the fielddeveloped by electromagnet 20 and then it is subjected to the focusingaction of the field developed by permanent magnet 24. As indicated bythe directions of the arrow-heads on flux lines 23 and 27 in Fig. 3, thefields associated with the respective magnets 20 and 24 are aligned inopposite directions. This will have little or no effect on the focusingof the two fields, however. The axial component of the magnetic fieldswill exert a small or negligible influence on the velocity of theelectrons, especially since the fields are short, and any effect due toone magnetic field will tend to be canceled by the other acting in theopposite direction. The combination radial and tangential components areessentially the forces responsible for focusing. Thus, axial electronswill pass straight through the magnetic center of lens 17, subject onlyto the minimal effect of the axial components of the two fields.Paraxial electrons will be subject to the effect of the axial componentto a like extent and will also be affected by the radial and tangentialcomponents in the usual manner, causing them to be focused according towell known principles of electron optics. It is noted, however, that theusual rotation to which paraxial electrons are subjected in passingthrough a magnetic focusing lens will be somewhat reduced since therotations imparted by the separate magnetic fields associated withmagnets 20 and 24 will be in opposite directions, according to thefamiliar right-hand rule.

In Fig. 4, there is shown a modified form of the invention in which themagnetic cylinder (24 of Figs. 2 and 3) has been replaced by a pluralityof magnetized bars or slugs 28 having their poles aligned in conformityand disposed at equi-spaced positions about the axis of the lens.

The portions of Fig. 4 bearing the numerals designated prime correspondto the portions of Fig. 2 having similar numerals. In cooperation withthe associated pole pieces, the bars or slugs 28 are essentially aseffective as the magnetized cylinder 24 of Fig. 3. This construction isalso less difiicult to manufacture and represents a cost reduction overthe cylindrical form.

It is seen from the foregoing description that I have provided animproved combination electromagnet-permanent magnet electron-beamfocusing lens of increased efficiency having a permanent magnet portionof reduced size and weight, thus representing a saving in the quantityof magnet material used.

While specific embodiments have been shown and described, it will, ofcourse, be understood that various modifications may be made withoutdeparting from the principles of the invention. The appended claims aretherefore intended to cover any such modifications within the truespirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electron beam focusing lens for cathode-ray tubes and the likecomprising, in combination, first, second and third generally annularpole pieces axially aligned in that order in spaced relation with oneanother to provide axial gaps therebetween, electromagnetic means fordeveloping a first axial field across the gap between said first andsaid second pole pieces, and permanent magnet means for developing asecond axial field in polar opposed relation to said first field acrossthe gap between said second and third pole pieces.

2. An electron beam focusing lens for cathode-ray tubes and the likecomprising, in combination, electromagnetic focusing means including agenerally toroidal coil, and north and south pole pieces, permanentmagnet focusing means also including north and south pole pieces, andmeans for axially aligning said first and second focusing means in polaropposed relation, one of said pole pieces being common to both saidfocusing means.

3. An electron beam focusing lens for cathode-ray tubes and the likecomprising, in combination, electromagnetic focusing means including agenerally toroidal coil and a housing of low reluctance materialenclosing said coil except for a generally annular gap therein, andpermanent magnet focusing means including a pair of spaced pole piecesbridged by permanently magnetized material, one of said pole piecesincluding a generally annular ring of low reluctance material and theother including a portion of said housing, said electromagnetic focusingmeans and said permanent magnet focusing means being arranged in polaropposed relation to one another.

4. An electron beam focusing lens for cathode-ray tubes and the likecomprising, in combination, a first pair of spaced generally annularpole pieces defining a first axial gap therebetween, a second pair ofspaced generally annular pole pieces including one of said first pair ofpole pieces and defining a second axial gap therebetween,electromagnetic means including a generally toroidal coil and a housingfor said coil for developing an axial magnetic field across said firstgap, and permanent magnet means for developing an axial magnetic fieldacross said second gap in polar opposed relation to the field acrosssaid first gap.

5. An electron beam focusing lens for cathoderay tubes and the likecomprising, in combination, electromagnetic focusing means including agenerally toroidal coil, means for supplying an operating potential tosaid coil, and a pair of pole pieces defining a first cylindrical gap,permanent magnet focusing means including a generally annular permanentmagnet assembly having a pair of pole pieces defining a secondcylindrical gap, and means for axially aligning said electromagneticfocusing means and said permanent magnet focusing means in polar opposedrelation, said means including a pole piece common to both focusingmeans.

6. An electron beam focusing lens for cathode-ray tubes and the likecomprising, in combination, electromagnetic focusing means including agenerally toroidal coil and a low reluctance housing enclosing said coilexcept for an annular gap therein, and permanent magnet focusing meansaxially aligned in polar-opposed relation to said electromagnetic means,said permanent magnet means including a pair of spaced pole piecesbridged by a generally cylindrical permanent magnet, one of said polepieces including a generally annular ring of magnetic material and theother including a portion of said housing.

7. An electron beam focusing lens for cathode-ray tubes and the likecomprising, in combination, a generally toroidal coil, means forenergizing said coil to develop a magnetic field, a low reluctancehousing enclosing said coil except for an annular gap therein concentricwith said coil, a generally cylindrical permanent magnet axially alignedwith said coil in polar opposed relation to the field developed in saidcoil and abutting against said housing, and a generally annular polepiece axially aligned with said permanent magnet and abutting againstthe end of said permanent magnet remote from said housing.

8. An electron beam focusing lens for cathode-ray tubes and the likecomprising, in combination, a generally toroidal coil, means forenergizing said coil to develop a magnetic field, a low reluctancehousing enclosing said coil except for an annular gap therein concentricWith said coil, a plurality of permanent magnet slugs in parallelrelation radially displaced about an axis aligned with the axis of saidcoil, in polar opposed relation to the field developed in said coil andabutting against said housing, a generally annular pole piece axiallyaligned with said permanent magnets and abutting against the ends ofsaid per "mm magnets remote from said housing.

*5 9. "In combination with a cathode-ray tube, three axially aligned andaxially spaced generally annular pole pieces, electromagnetic means fordeveloping a first axial field across'fa gap between the center polepiece and a first end pole piece, and permanent magnet means fordeveloping a second axial field in polar opposed relation to said firstfield across a gap between said outer pole piece and said other" endpole piece.

{'10, In combination with a cathode-ray tube, a first pair of spacedannular pole pieces defining a first axial gap therebetween, a secondpair of spaced annular pole pieces including one of said first pair ofpole pieces and defining a second axial gap therebetween,electromagnetic means for developing a magnetic field across said firstgap, and

permanent magnet means for developing a magnetic field across saidsecond gap in polar opposed relation to the field across said first gap.

References Cited in the file of this patent UNITED STATES PATENTS2,219,193 Mynall Oct. 22, 1940 2,306,875 Fremlin Dec. 29, 1942 2,533,687Quam Dec. 12, 1950 2,563,116 Hultgren Aug. 7, 1951

